This invention relates to the placement of instruments in boreholes in the ground. For example, the invention can be applied when placing water-quality instrumentation and sampling apparatus in a water monitoring well, and the invention will be described as it relates to that application.
In this type of operation, a pipe is passed down from the ground surface into the borehole. The pipe serves as a housing for one or more tubes, which lead down to the sampling points, generally under water, in the borehole.
Often, the pipe is, say, half the diameter of the borehole. The need arises for sealing the annulus around the pipe, to prevent an up/down flow of water.
One of the parameters the designer should have in mind when designing a water sampling and instrumentation system is the need to make sure that the water drawn up when taking a sample actually does come from the depth indicated. If water is allowed to pass easily up and down the borehole, more readily than water can naturally flow up/down in the surrounding ground, a reading taken at depth A might actually be measuring water that comes from depth B.
The requirement arises, therefore, for a means for sealing off the sampling point at a certain depth, from water at other depths. Conventionally, packing seals of various types and designs have been used and proposed for this purpose.
The seal, or packer, must expand to fill the annular (radial) gap between the pipe and the borehole, after the pipe has been lowered into the borehole. The use of bentonite for sealing pipe-to-borehole gaps has long been favoured, because bentonite expands upon immersion in water. The bentonite is in a dry condition when it is lowered into position in the borehole, and then the bentonite is left to expand, as it becomes exposed to, and is penetrated by, the water in the borehole.
The present invention provides a packing or sealing system, which is aimed at enabling the seal material, such as bentonite, to be placed around the pipe, at the desired depth in the borehole, in a manner whereby it is considerably easier to achieve the desired security of seal, with less skill and care required of the installer, than has been the case with conventional packing systems.
It is recognised that the system of the invention can also serve as a means for installing materials other than expandable or inflatable sealing materials, such as sand/gravel or other porous materials, where these are needed, accurately at a specified depth, in a borehole. Where the pipe has a sampling port in the pipe, for example, it is conventional for a screen of sand or gravel to be packed around the port. Unlike the expandable sealing materials, the sand/gravel is not intended to completely fill the borehole.
In the context of the invention, the term xe2x80x9cpackerxe2x80x9d should be construed as including not only a sealing packer that uses expandable sealing material such as bentonite, but also a filter-screen packer that uses sand or gravel.
In the invention, the packer is not made up on-site, but is pre-manufactured, in a factory. The packer comprises an inner sleeve and an outer sleeve, and the packing material is located in the annular chamber between the two sleeves.
The packer is shipped to the field site for assembly onto the pipe, and for installation into the borehole. As such, at the time the packer is manufactured, the packer does not have the benefit of the presence of the pipe. Therefore, the designer must provide some means whereby the packer can be physically supported to the degree of robustness required for handling and transport of the packer.
In one aspect of the invention, the pre-manufactured packer has a structural-robustness-member incorporated into the structure thereof.
The structural-robustness-member may comprise the inner sleeve of the packer, the inner sleeve in that case being made of rigid material, such as metal.
The structural-robustness-member may comprise a shipping-tube, e.g of cardboard, which fits inside the inner sleeve. In this case, the inner sleeve may be thin plastic mesh.
Another aspect of the invention lies in a procedure for making and installing an expanding sealing packer. Here, the bentonite (or other water-expandable material) is compressed to sufficient pressure that the material coheres, and the material is thereby compression moulded into the shape of an annular-ring.
The annular-ring, or several annular-rings, are placed inside a sleeve, preferably of flexible plastic mesh material, to form the packer.
The annular-rings being rigid and robust, the rings themselves may serve to give the packer the required degree of structural robustness.